Currently, a wide variety of materials and construction techniques may be selected for the construction of single-story and multi-story buildings. For example, wood is often used for the framing of residential buildings; and, while wood may often be used during the construction of industrial, commercial, and institutional buildings, it is used primarily for temporary purposes and even then is often more costly than other materials. However, wood is not often suitable for the framing of industrial, commercial, and institutional buildings because wood is usually not strong enough to withstand the amount of load normally associated with industrial, commercial, and institutional buildings, and because wood is not particularly fire resistant.
Steel is often used as framing for industrial, commercial, and institutional buildings, particularly for long-span, single-story structures or for short span, single-story and multi-story buildings where either fireproof structures are not required or an adequate sprinkler system is provided. Unprotected steel begins to lose its strength at about 700.degree. F. and, therefore, the steel used in steel frame construction may warp, twist and fail in the event of a serious fire in the building.
Where structures must be fireproofed, construction using reinforced concrete will generally be more economical, except where long spans are required, in which case steel framing, fireproofed with thin gunite or other light weight material, may be lower in cost. Reinforced concrete construction provides full fireproof construction. Also, the use of precast concrete elements, which may be cast off site or may be cast at the building site itself, reduces the amount of time and labor required to frame a building. Thus, precast concrete elements are used increasingly for such structural members as beams and columns, as well as for walls, floors, and roofs. These precast concrete elements are typically reinforced with steel rods, wire, or cable which may be prestressed or poststressed for additional strength.
The precasting of concrete elements, such as slabs, beams, columns, walls, and/or partitions, requires the construction of either metal or wood forms. The forms typically hold the reinforcing steel rods, wire, or cable in place while concrete is poured into the form. To reduce the number of connections (i.e., joints) between precast concrete elements, these elements should generally be cast as large as can be properly handled. However, some connections will be needed between precast concrete elements, and these connections must transfer moments, torsion, shear, and/or axial loads from one precast concrete element to another.
The integrity of a building erected by the use of precast concrete elements depends upon the adequacy of the design of the connections between the building's precast concrete elements. These connections may be made by the use of pins, clips, keys, welding, or by any number of other methods, and must withstand stresses due to a building's live and dead loads. Live loads result from items which are typically not a permanent part of the building structure, such as machinery, office equipment, people, snow, and the like. Dead loads are due to the weight of the building structure, partitions, and permanent equipment. Bolts have also been used to connect two concrete elements together; however, these bolts have been arranged to transfer primarily shear loads from one concrete element to another rather than to transfer primarily tension loads between concrete elements. Even though the use of precast concrete elements reduces the amount of time and labor required to frame a building, the current methods of providing connections between precast concrete elements still is unnecessarily labor intensive and requires a substantial amount of time to implement.
When buildings are erected in seismic zones, i.e. those geographical zones in which earthquakes occur, the connections between the concrete elements of a building experience additional loading due to vibrations produced by earthquakes. The earthquake effects on a building are generally represented as horizontal forces. A common rule used in the construction of a building is that the building frame should resist such horizontal forces equal to approximately one-tenth of the dead load supported by the building frame. In some cases, the building frame is designed to resist such horizontal forces equal to approximately one-tenth of the dead load plus a portion of the live load supported by the building frame. Even though improvements have been made in the techniques used to construct buildings so that they can better withstand earthquakes, such construction techniques still do not ensure that buildings can adequately withstand the forces exerted on them by earthquakes.